Each year in the United States, about 100,000 workers may be exposed to low doses of neutrons. In addition, calculations indicate that some of the ~300,000 U.S. airline crew members will receive more than the maximum permissible dose for non-radiation workers, about half the dose equivalent coming from neutrons. In all cases, the majority of the neutron dose is delivered in the keV range, where the secondary protons set in motion by these neutrons in tissue have a very short range. On average, the important energy range for deposition of dose is from about 30 to 500 keV and about half the dose comes from energies below 140 KeV. Conventional theory predicts decreasing RBE with decreasing neutron energies below about 200 keV. However, recent evidence suggests that there may be significant biological response at low (sOft) neutron energies. In the neutron range below 100 KeV the results of the Harwell group, both for dicentric chromosomal aberration yields in human lymphocytes, and for other end points in rodent cells, suggest comparable yields to those at a few hundred keV. In terms of radiation protection issues addressed in the current proposal, a significant increase in the biological effectiveness of neutrons from the hundreds of keV to the tens of keV range, would result in an increase in the quality factor appropriate for most occupational exposure situations. The objective of the present proposal is to determine the RBE of soft neutrons using biological test systems closely related to late effects. First, oncogenic transformation with freshly explanted Syria hamster embryo cells (SHE), provides good quantitation of risk to soft neutrons after either acute or fractionated doses. Second, transformation studies with a permanent line of human uroepithelial cells (SV-HUC) that represents a qualitative system of human oncogenesis will be used. An established source of soft neutrons available as monoenergetic neutrons of very low energy is available at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The biological systems, especially the SHE cell transformation assay, are sufficiently sensitive to allow RBE estimates down to about 5 cGy of neutrons, if the soft neutrons prove to be biologically effective. RARAF is the only source of soft neutrons presently available in the world able to answer one of the most important unanswered questions in radiobiology. In terms of radiation protection the data obtained in the proposed research will represent one vital link in the chain of information necessary for the reassessment of somatic hazards from ionizing radiation.